Let's be real...I only do this when I'm flying Spirit and I don't want to pay for an extra bag. I hate Spirit, but for a short trip it's worth the cheap airfare (in my opinion). Here are a few things I do to fit everything I need in one backpack.
1.Plan your outfits before hand. Only pack what you absolutely need and choose versatile pieces that can be worn more ways than one.
2. Put your shoes at the bottom of the bag. Wear your bulkiest shoes on the plane.
3. Stuff shoes with small pieces of clothing.Socks & undergarments.
4. Roll all of your clothes up tight.I swear this saves room.
5. Place curling irons/straighteners upright along one side of the bag. Pretty much takes up 0 space.
6. Carry your jackets or bulky piece of clothing. Use it as a pillow on the plane!
7. Make sure your wallet and essentials are at the top of your bag. So you don't have to go digging when you're in the security line or want to grab snacks at your gate.